An air conditioner for a vehicle is an apparatus for cooling or heating the interior of the vehicle by cooling or heating through the process of introducing outdoor air into the interior of the vehicle or circulating indoor air of the vehicle. Such an air conditioner for a vehicle includes an evaporator for cooling the inside of an air-conditioning case; a heater core for heating the inside of the air-conditioning case; and a mode converting door for selectively blowing the air cooled by the evaporator or heated by the heater core toward parts of the interior of the vehicle.
According to independent structures of a blower unit, an evaporator unit and a heater core unit, such an air conditioner is classified into a three-piece type air conditioner in which the blower unit, the evaporator unit and the heater core unit are disposed independently, a semi-center type air conditioner in which the evaporator unit and the heater core unit are embedded in the air-conditioning case and the blower unit is mounted separately, and a center-mounting type air conditioner in which the three units are all embedded in the air-conditioning case.
In order to increase effectiveness of the interior space of a vehicle, compact-sized and miniaturized air conditioners for vehicles are being demanded. In response to such demand, recently, development of a center-mounting type air conditioner in which a blower unit, an evaporator unit and a heater core unit are integrated is being promoted. As an example, such a center-mounting type air conditioner for a vehicle is illustrated in FIGS. 1 and 2.
The center-mounting type air conditioner 1 illustrated in FIGS. 1 and 2 includes: an air-conditioning case 10 in which a blower 20 is mounted at an inlet and a plurality of air outflow ports 12a to 12e formed at an outlet thereof; an evaporator 2 and a heater core 3 embedded in the air-conditioning case 10; one or more temperature-adjusting doors 14 for adjusting the degree of opening of a cold air passageway P1 and a warm air passageway P2 inside the air-conditioning case 10; and a plurality of mode doors 13 for adjusting the degree of opening of the air outflow ports 12a to 12e. 
Moreover, the blower 20 includes: a scroll case 21 disposed at the inlet of the air-conditioning case 10; a centrifugal fan 25 rotatably mounted inside the scroll case 21; an intake duct 30 which is mounted at one side of the scroll case 21 and has inside and outside air inflow ports 31 and 32, which are opened and closed by an intake door 33; an inlet ring 22 which is formed at one side of the scroll case 21 facing the intake duct 30 to guide the air induced through the inside and outside air inflow ports 31 and 32 toward the inside of the centrifugal fan 25; and a motor 27 mounted on the opposite side of the inlet ring 22 of the scroll case 21 to operate the centrifugal fan 25.
The intake door 33 opens the inside air inflow port 31 in the inside air inflow mode so that only inside air flows in but opens the outside air inflow port 32 in the outside air inflow mode so that only outside air flows in.
Moreover, an air filter 35 for removing foreign matters contained in the air is mounted inside the intake duct 30.
In the meantime, the intake door 33 is configured as a flat type door, and in this instance, a rotary shaft of the intake door 33 is arranged to be at right angles to a rotary shaft of the centrifugal fan 25.
Therefore, when the centrifugal fan 25 of the blower 20 is operated, low pressure is formed inside the centrifugal fan 25 by rotation of the centrifugal fan 25 so that air is inhaled in the axial direction of the centrifugal fan 25 through the inlet ring 22 and is blown in the radial direction of the centrifugal fan 25, and then, the blown air is discharged to the inside of the air-conditioning case 10 through an outlet 23 of the scroll case 21.
Continuously, the air discharged to the inside of the air-conditioning case 10 is cooled while passing through the evaporator 2, and then, bypasses the heater core 3 by the temperature-adjusting door 14 to flow in a cold air state or passes the heater core 3 to be converted into warm air through heat-exchange. After that, the air is discharged to the interior of the vehicle through the air outflow ports 12 opened according to the air-conditioning modes so as to cool or heat the interior of the vehicle.
However, the conventional blower 20 has a disadvantage in that it requires lots of driving power to raise heating performance when a heater is operated in cold weather, such as winter season. That is, in cold weather such as winter season, the air conditioner is generally set as the outside air inflow mode not only to maintain the interior of the vehicle in a heated state but also to prevent frost on windows. Therefore, the conventional air conditioner requires lots of driving power to heat the interior of the vehicle and is deteriorated in heating performance because only cold air outside the vehicle is introduced through the outside air inflow port 32.
In order to solve the above problems, as shown in FIG. 3, the intake door 33 has an inside air inflow structure to raise heating performance by partially introducing inside air in the outside air inflow mode. Referring to FIG. 3, the intake door 33 includes an auxiliary inside air passageway 33a, a rotary shaft 33b and an auxiliary door 34 mounted on the rotary shaft 33b of the intake door 33 for opening and closing the auxiliary inside air passageway 33a. 
Therefore, the auxiliary door 34 closes the auxiliary inside air passageway 33a in the inside air inflow mode, and opens the auxiliary inside air passageway 33a by negative pressure generated by operation of the centrifugal fan 25 in the outside air inflow mode. Therefore, when the heater is operated in the outside air inflow mode, outside air introduced through the outside air inflow port 32 and inside air introduced through the auxiliary inside air passageway 33a are mixed together, and then, the mixed air is introduced into the air-conditioning case 10 so as to reduce driving power required for heating and raise heating performance.
However, the conventional air conditioner for the vehicle has a disadvantage in that manufacturing processes and manufacturing costs are increased due to an increase of the number of components because it requires additional components, such as the auxiliary door 34 additionally mounted on the intake door 33 to inhale inside air in the outside air inflow mode.